1. Field of the Invention
The present invention relates to an oxygen sensor for detecting oxygen in a gas to be measured, such as exhaust gas from an internal combustion engine.
2. Description of the Related Art
A known oxygen sensor includes an oxygen detection element assuming the form of a hollow rod which is closed at a front end, and having electrode layers formed on the inner and outer surfaces thereof. In an oxygen sensor of this type, while the atmosphere serving as a reference gas is introduced into an oxygen detection element such that the inner surface (internal electrode layer) of the element is exposed to the reference gas, the outer surface (external electrode layer) of the oxygen detection element is exposed to exhaust gas. As a result, an electromotive force is induced by the oxygen concentration cell effect, according to the difference in oxygen concentration between the inner and outer surfaces. This electromotive force induced by the oxygen concentration cell effect is led out from the internal and external electrode layers through lead wires and serves as a detection signal indicative of oxygen concentration in the exhaust gas.
FIG. 12 of the accompanying drawings shows a conventional metallic internal-electrode connection member (metallic terminal member) 23xe2x80x2 to be installed into a hollow portion 2a of such an oxygen detection element 2 so as to establish electrical connection with an internal electrode layer formed on the inner wall surface of the hollow portion 2a. The conventional metallic internal-electrode connection member 23xe2x80x2 includes the following integrally formed portions: a connector 23axe2x80x2 to be connected to a lead wire; a main body portion 23cxe2x80x2 to come into contact with the inner wall surface of the hollow portion 2a of the oxygen detection element 2; a lead portion 23b xe2x80x2 for connecting the connector 23axe2x80x2 and the main body portion 23cxe2x80x2; and a heating member holder portion 23dxe2x80x2 for firmly holding a heating member which is disposed within the hollow portion 2a for heating the oxygen detection element 2.
The main body portion 23cxe2x80x2 of the conventional metallic internal-electrode connection member 23xe2x80x2 is formed by bending into a cylindrical form a sheet member which has a plurality of contact portions 23exe2x80x2 formed into a saw-toothed form and arranged at opposite side edges thereof in a staggered manner. Substantially the entire outer circumferential surface of the main body portion 23cxe2x80x2 is brought into contact with the inner wall surface (internal electrode layer) of the hollow portion 2a of the oxygen detection element 2, whereby electrical continuity is established and the main body portion 23cxe2x80x2 is axially positioned relative to the hollow portion 2a. 
In order to reliably position the metallic internal-electrode connection member 23xe2x80x2 in the axial direction relative to the hollow portion 2a and to establish reliable contact and electrical connection between the metallic internal-electrode connection member 23xe2x80x2 and the internal electrode layer, the outside diameter of the cylindrical main body portion 23cxe2x80x2 is rendered greater than the inside diameter of the hollow portion 2a of the oxygen detection element 2a. Thus, as shown in FIG. 12, when the metallic internal-electrode connection member 23xe2x80x2 is to be installed in the oxygen detection element 2, the main body portion 23cxe2x80x2 is inserted under pressure into the hollow portion 2a while substantially the entire outer circumferential surface of the main body portion 23cxe2x80x2 is squeezed radially. As a result, the resistance of insertion tends to increase, potentially raising a problem in assembly. Particularly, since a plurality of contact portions 23exe2x80x2 formed into a saw-toothed form are arranged at opposite sides in a staggered manner, the resistance of insertion tends to occur intermittently. As a result, in some cases, an upper portion (the base-end side relative to the insertion direction) of the metallic internal-electrode connection member 23xe2x80x2 suffers plastic deformation, such as crushing, bending, or buckling. In order to prevent such plastic deformation, a relevant jig may be employed; however, this involves additional work and causes an increase in cost.
It is an object of the present invention to provide a sensor structure which reduces the resistance of insertion in the course of insertion of a metallic terminal member into the hollow portion of an oxygen detection element so as to enable smooth assembly, and such that portions of the metallic terminal member become less susceptible to plastic deformation.
Accordingly, an oxygen sensor of the present invention comprises an oxygen detection element assuming the form of a hollow rod which is closed at one end, and having an electrode layer formed on at least the inner surface thereof; and a metallic terminal member connected electrically to the electrode layer. The oxygen sensor is characterized in that:
the metallic terminal member includes an attachment portion having a substantially circular cross section, which is disposed within a hollow portion of the oxygen detection element; and
the attachment portion is disposed such that, as observed in cross section, the attachment portion is in contact with the inner wall surface of the hollow portion of the oxygen detection element at opposite sides thereof located along a predetermined direction (hereinafter called the direction of contact), and a gap is formed between the attachment portion and the inner wall surface of the hollow portion of the oxygen detection element at opposite sides thereof located along a direction intersecting the direction of contact (hereinafter called the direction of gap formation).
As described above, according to the present invention, the attachment portion of the metallic terminal member is in contact with the inner wall surface of the hollow portion of the oxygen detection element, directly or indirectly via another member, at opposite sides thereof located along the direction of contact. Also, a gap is formed between the attachment portion and the inner wall surface at opposite sides of the attachment portion located along the direction of gap formation. Thus, only a portion of the outer circumferential surface of the attachment portion is in contact with the inner wall surface of the hollow portion to thereby establish electrical continuity therebetween. Specifically, the attachment portion is in contact with the inner wall surface of the hollow portion at two or more contact points to thereby establish electrical continuity therebetween. Thus, in the course of insertion of the metallic terminal member into the hollow portion of the oxygen detection element, the resistance of insertion decreases, whereby assembly can be performed smoothly, and portions of the metallic terminal member become less susceptible to plastic deformation, such as crushing, bending, or buckling.
Preferably, the attachment portion of the present invention, as observed in cross section, has an opening formed at a portion of the circumference thereof and includes a direction change portion, which is located opposite the opening with respect to the center axis of the hollow portion of the oxygen detection element; and
edge portions located at opposite sides of the opening and the direction change portion are in contact with the inner wall surface of the hollow portion of the oxygen detection element, directly or indirectly via another member, and a direction extending between the direction change portion and one of the edge portions located at opposite sides of the opening is the direction of contact. Thus, the attachment portion can be manufactured through bending of a sheet member. Also, the attachment portion can be designed and machined with high accuracy so as to establish the above-mentioned state of contact and gap formation. By virtue of the opening, the attachment portion is inserted under pressure into the hollow portion while being elastically deformed such that the edge portions located at opposite sides of the opening are squeezed radially inward, whereby insertion is performed smoothly. Further, the attachment portion is in contact with the inner wall surface of the hollow portion of the oxygen detection element at the three portions-edge portions located at opposite sides of the opening, and the direction change portionxe2x80x94to thereby be fixedly positioned in a stable manner.
Preferably, the attachment portion of the present invention, as observed in cross section, includes parallel portions which are located opposite each other along the direction of gap formation. Thus, in the course of design and machining, the state of gap formation can be established easily and reliably, thereby facilitating installation.
Preferably, the attachment portion of the present invention is inserted in the hollow portion of the oxygen detection element in such manner as to be elastically deformed radially inward; and
when the attachment portion is removed from the hollow portion while being elastically restored, as observed in cross section, a maximum distance between opposite points of the attachment portion located along the direction of contact and as projected on a line passing through the center of width of the opening and the center of the hollow portion is equal to or greater than the inside diameter of the oxygen detection element. By virtue of a resilient force associated with squeezing of the engagement portion in the direction of contact, the attachment portion is brought into reliable and secure contact with the inner wall surface of the hollow portion of the oxygen detection element, directly or indirectly via another member.
Preferably, according to the present invention, as observed in a longitudinal section which includes the opening and the center axis of the metallic terminal member, the edge portions located at opposite sides of the opening extend linearly in the direction of the axis of the hollow portion of the oxygen detection element. In contrast with the conventional type in which a plurality of contact portions formed into a saw-toothed form are arranged at opposite sides in a staggered manner with resultant intermittent occurrence of the resistance of insertion, the resistance of insertion is reduced, thereby enabling further smooth insertion of the metallic terminal member.
Preferably, according to the present invention, a diameter reduction portion is formed on the attachment portion at the front side relative to the insertion direction of the attachment portion into the hollow portion of the oxygen detection element; and, as observed in the longitudinal section which includes the opening and the center axis of the metallic terminal member, the diameter reduction portion includes a portion (hereinafter called a first portion) which is located adjacent to the edge portions located at opposite sides of the opening and which decreases in diameter continuously or stepwise at the front-end side relative to the insertion direction. The metallic terminal member is inserted into the hollow portion of the oxygen detection element while the first portion is guided by the hollow portion, and insertion of the edge portions located at opposite sides of the opening follows insertion of the first portion. Thus, the resistance of insertion in the course of assembly is further reduced, and the inserted metallic terminal member is in reliable and secure contact with the inner wall surface of the hollow portion.
Preferably, according to the present invention, at a contact portion between the attachment portion of the metallic terminal member and the inner wall surface of the hollow portion of the oxygen detection element, the radius of curvature of the outer circumferential surface of the attachment portion is smaller than that of the inner wall surface. Thus, the area of contact at the contact portion is reduced, thereby reducing the resistance of insertion of the attachment portion in the course of assembly.
Preferably, according to the present invention, a counter-bore portion is formed in a rear-end opening portion of the hollow portion of the oxygen detection in a diameter-expanded manner so as to receive the attachment portion, directly or indirectly via another member. This structure prevents plastic deformation of the attachment portion, which would otherwise occur in association with insertion into the hollow portion of the oxygen detection element, and play or coming-off of the attachment portion, which would otherwise occur due to exposure to repeated vibration. Thus, the metallic terminal member can be fixedly positioned within the oxygen detection element in a smooth and reliable manner.
Preferably, the diameter reduction portion may include a portion (hereinafter called a second portion) which is located opposite the first portion with respect to the center axis of the hollow portion of the oxygen detection element and which, as observed in the longitudinal section which includes the opening and the center axis of the metallic terminal member, decreases in size continuously or stepwise toward the front-end side relative to the insertion direction. In the course of insertion of the metallic terminal member into the hollow portion of the oxygen detection element, the second portion contributes to further reduction in the resistance of insertion of the diameter reduction portion and to reliable, secure attachment of the metallic terminal member onto the inner wall surface of the hollow portion.
Preferably, the second portion of the diameter reduction portion may have a cut formed therein extending from the front end thereof relative to the insertion direction toward the base-end side relative to the insertion direction. The cut formed in the second portion contributes to a great reduction in the resistance of insertion of the attachment portion, particularly at the beginning of insertion. A reduction portion may be formed at the bottom of the cut such that the width along the circumferential direction of the inner wall surface of the hollow portion of the oxygen detection element decreases continuously toward the base-end side relative to the insertion direction. The reduction portion formed at the bottom of the cut contributes to a great reduction in the resistance of insertion, particularly at the end position of the diameter reduction portion in the course of insertion of insertion.
Preferably, the outline of the cut in the present invention as projected on a longitudinal section which includes the bottom point of the cut and the center axis of the metallic terminal member may assume a form so as to approach the inner wall surface of the hollow portion toward the base-end side relative to the insertion direction. The outline assuming this form may be realized by, for example, formation of a sub-cut in the second portion in such manner as to extend from the bottom of the cut toward the base-end side relative to the insertion direction. In any case, since the outline assumes such a form as to gradually approach the inner wall surface of the hollow portion toward the base-end side relative to the insertion direction, the second portion is rounded at the base-end side thereof relative to the insertion direction, and the rounded portion comes into contact with the inner wall surface of the hollow portion, thereby further reducing the resistance of insertion of the diameter reduction portion in the course of assembly. Also, chipping of the electrode layer becomes less likely to occur.
As a result of forming the sub-cut in the second portion, the outline of the cut includes an inflection point at which the form of a radially inward convex at the front-end side thereof relative to the insertion direction changes to the form of a radially outward convex at the base-end side thereof relative to the insertion direction. The outline may include a region in which the rate of change gradually decreases toward the inner wall surface of the hollow portion, where the rate of change is represented by a fraction having a denominator indicative of the amount of change in the direction of insertion and a numerator indicative of the amount of change in a radially outward direction perpendicular to the direction of insertion.
By forming the sub-cut in the second portion of the diameter reduction portion as mentioned above, the outline of the cut includes an inflection point, whereby the outline of the cut includes a rate-of-change gradual-decrease region. By forming the rate-of-change gradual-decrease region, at the base-end side of the second portion relative to the insertion direction, the amount of approach in a radially outward direction (the amount of approach to the inner wall surface of the hollow portion of the oxygen detection element) decreases gradually as the amount of insertion of the diameter reduction portion increases. Thus, in the course of assembly, the resistance of insertion of the diameter reduction portion further decreases. Also, while the second portion maintains a smooth outline, the rate of dimensional change in the axial direction (the direction of insertion) can be rendered great as compared to the rate of dimensional change in a radial direction. Thus, the size of the metallic terminal member can be reduced, whereby the oxygen detection element and the oxygen sensor can be formed to a compact size.